The present invention generally relates to distribution series capacitor control systems. More particularly, the present invention relates to sub-harmonic and ferroresonance detection systems for preventing damage to the distribution series capacitor control systems and related equipment.
The quality of power distributed through modern electrical distribution systems continues to be an issue concerning operators of large systems. One such power quality problem is known as voltage flicker. Voltage flicker is a voltage dip that is of a magnitude sufficient to have an objectionable effect on other loads connected to the same circuit. The disturbance may be experienced as only blinking lights, but the magnitude and the frequency of the occurrences determine flicker's impact on system users.
FIG. 9 illustrates a common voltage flicker scenario. Flicker-producing loads 910 on system 900 are typically caused by large motors, welders, or arc-furnaces. These loads are characterized by high inrush currents of relatively short duration, as experienced in the starting of a motor. The motor's inrush current is typically of a low power factor, and causes a voltage dip of increasing magnitude along the feeder up to the point of the load's connection. This causes voltage flicker problems between the load and the source 920, which, when severe enough often leads to a complaint 930.
The distribution series capacitor 940 has long been recognized as a cost-effective solution to these types of flicker problems. Unfortunately, distribution-class electrical power lines equipped with a distribution series capacitor are subject to two distinct and damaging phenomena, ferroresonance involving transformers, and selfexcitation of motors during starting. Ferroresonance is an often severe and rapidly building oscillatory overvoltage condition caused by system non-linearities that can appear when power transformer cores saturate. These non-linearities interact with the series capacitor to produce a low-frequency resonant condition, often in response to large inrush currents following breaker operations. Self-excitation of induction motors is a potentially damaging condition that can occur on the same system. The term "self-excitation" refers to sub-harmonic oscillations that may occur in an electric supply circuit that includes series capacitors. The sub-harmonic oscillations result from the interaction between the series capacitors and an induction motor when the motor is in the process of starting. These oscillations are typically characterized by motor starting problems and sustained overcurrent conditions.
When ferroresonance occurs, immediate action must be taken to prevent damage to other equipment. Ferroresonance is a rapidly occurring, high magnitude, and low frequency oscillation capable of reaching power system voltage levels of 100-200% above normal for brief periods. When self-excitation occurs, low-frequency oscillations are produced as the motor starting sequence fails. The motor will search for the proper operating frequency, which will cause large current surges as the shaft acceleration alternates.
Prior attempts to detect these problems included analog filtering and simple timed-level detection methods to make system decisions. Since these detection systems do not differentiate between these two types of sub-harmonic conditions, they can not adequately safeguard the system while simultaneously maximizing efficiency. Further. the prior systems were not designed to act quickly enough to avoid severe ferroresonance conditions.